The tracks on track-type earthmoving vehicles work in an adverse, highly abrasive environment. As a consequence, the various track components wear out and must be replaced from time to time. Such track typically consists of an endless track chain and ground engaging track shoes. The track shoes are normally bolted onto the link assembly and are therefore readily replaceable independently of the other components of the track. Typical track chains, on the other hand, generally consist of links, pins, and bushings, as well as seals, thrust rings and lubricants. Such components of the track chain are assembled by pressing the respective ends of the links onto the pins and bushings by means of a high-capacity track press. Such a high-capacity track press typically produces 100 tons of force and is also required to disassemble such chains. Accordingly, replacement of nay one of the particular components of the chain that wears out entails the removal of the track from the vehicle and its portage to a track service facility equipped with a track press. This whole procedure is quite expensive and time consuming. It is therefore desirable and advantageous to minimize the number of times such track chain is disassembled during its service life. In the past, this has been difficult to achieve because each of the various chain components had its own particular wear life that could vary significantly from the wear lives of the other components. In short, one component would wear out long before the other components would. Some fifteen years ago, internal pin and bushing wear caused the pin and bushing to be the shortest lived components. This was corrected by the introduction of sealed and lubricated track that virtually eliminated internal pin and bushing wear. This increased track life to a point to where the bushing now wears out first due to external wear.
External track bushing wear occurs because bushings function as drive members. Such track bushings engage the toothed drive sprocket of the vehicle and the large locomotive force form the vehicle is transmitted form the drive sprocket into the track through the bushings. External bushing wear principally occurs due to scrubbing contact with the teeth of the drive sprocket as the bushing engages and disengages the drive sprocket. This wear occurs only at one spot or on one side of the bushing as the bushing remains stationary or fixed relative to the links to which they are attached during operation. Additionally, the typical environment of such vehicles contains considerable abrasive materials such as sand, dust, dirt and mud. Because of all of this, that portion of the external surface of the bushing which engages the sprocket is subject to a high degree of wear, while the rest of the external surface of the bushing receives little or no wear at all. In order to obtain additional wear life form the bushing, a procedure known as turning the bushings has been commonly employed. Bushing turning, unfortunately, also requires the undesirable disassembly of the track chain.
It has been recognized for some time that external bushing life could be extended by making the bushing rotatable. One such design is disclosed in U.S. Pat. No. 3,492,054 for Track Hinge Joints With Rotating Bushings that issued Jan. 27, 1970 to R. L. Boggs et al. and assigned to the assignee hereof. Such rotating bushing design was not commercialized, however, because of other problems it introduced. One such problem was the loss of structural rigidity in the track chain. Such loss is due to the fact that the bushing is no longer secured to the links. The ensuing flexure may lead to an excessive amount of end play in the joint beyond the capacity of the seals to seal, resulting in the loss of lubricant. Any loss of lubricant is critical because of the limited supply of lubricating oil contained in the track joint. This problem is magnified in the Boggs et al. design because the joint has twice the number of seals (four versus two). A failure of any one of such seals will result in early joint failure. Another problem was in the use of the track link itself as the bearing for the pin for purposes of track articulation. The link lacks sufficient hardness and other material characteristics to serve as a bearing for an adequate service life period.
The solution to the above problems is hampered by a third problem, which is inability to change certain dimensions of the track. One such dimension is the rail-to-rail width or gauge of the track. Like railroad track, the track links provide a pair of rail surfaces on which the wheels or rollers of the vehicle ride. The rail width or gauge for any particular vehicle is, for all practical purposes, permanently set and cannot be changed. This is because any change in rail gauge would require corresponding changes in the remaining components of the vehicle undercarriage and because the changed track would not be interchangeable with the track on existing vehicles.
The present invention is directed to overcoming the shortcomings of the prior attempts at providing a track with longer external bushing life.